This invention relates to regeneration of spent activated carbon, and more particularly to a fluidized bed type regenerator capable of continuously regenerating activated carbon in a fluidized state and with a high yield.
In continuous regeneration of spent activated carbon, it is known in the art to fluidize the spent carbon particles by an oxidative regeneration gas of about 500.degree. C. For example, U.S. Pat. No. 3,700,563 discloses a method of reactivating spent activated coal in a fluidized bed reactor by subjecting the spent activated coal to fluidization under continuous admission of flue gases while maintaining a constant fluidized bed temperature in the outlet region of the reactor by regulating the feed of spent activated coal to the reactor. In another reactivation process disclosed in British Pat. No. 686,294, deactivated charcoal from a desorber is introduced into a reactivation vessel or column in which progressively increasing temperatures prevail to provide a low temperature section, a median temperature section and a high temperature section. As the charcoal flows down the column, it is concurrently caused to contact high temperature flue gases to devolatilize the carbon particles and to remove adsorbed contaminants therefrom.
The usual procedure for cooling the hot carbon particles which have been withdrawn from the reactivator is either to wash them with water in a scrubber or to drop them directly into a water bath.
A serious problem encountered in the reactivation of spent activated carbon in a fluidized bed is that the activated carbon particles are reduced into fine powder due to attrition and collisions while being whirled in the fluidized bed during the desorption and reactivation processes, the fine powder leaving the reactivator or regenerator entrained in the exhaust gases. The entrainment of fine carbon powder in the exhaust gases gives rise to the necessity of gasifying the carbon powder in a combustion furnace upon disposing of the exhaust gases. In spite of the unignorably large amount of activated carbon which is reduced into powder during the reactivation, the conventional methods and apparatus have almost no provision for suppressing powdering of the activated carbon particles under reactivation treatment or for blocking fine carbon powder from being carried away out of the regenerator by the exhaust gases. Lacking such a provision, it is difficult to carry out the regeneration of spent activated carbon with a high yield (e.g., over 90 %), and a large consumption of combustion gas is required for gasifying the fine carbon powder in the exhaust gases. For these reasons, the regeneration of spent activated carbon in a fluidized bed has been considered to be disadvantageous from an economical point of view in spite of its inherent merit that a continuous operation is possible.
The present invention contemplates providing means for suppressing the powdering of the activated carbon particles and blocking the escape of fine carbon powder with exhaust gases, in the activated carbon regenerator of the construction disclosed in our copending application U.S. Ser. No. 472,097.
Although the finely powdered activated carbon causes trouble in the reactivation process, it is difficult to remove the fine powder beforehand from the spent activated carbon particles to be fed to the regenerator or to suppress the powdering completely throughout the reactivation process. It is also difficult to filter out and remove all of the fine carbon powder which is entrained in the exhaust gases. The fine powder is unavoidably brought into the regenerator along with the spent activated carbon particles and also produced in a certain amount during the regeneration process. The extent of powdering largely depends on the mechanical and physical properties of the activated carbon particles per se. As a matter of course, it is also influenced by the shape and type of the activated carbon particles to be treated and the interior construction of the regenerator. Activated carbon particles of complicated irregular shapes naturally produce a larger amount of fine powder than particles of simple round or spherical shapes. Regenerators of complicated construction or with exposed ends and edges are undesirable from the standpoint of production of fines. The powdering occurs even when the spent carbon particles are fed to the regenerator, for example, by a screw feeder. The regenerator is normally heated to a temperature of 600.degree. to 900.degree. C. so that, if the spent activated carbon particles were admitted into the regenerator in a wet state as usually is the case, they would break up or disintegrate due to thermal shock and impact resulting from abrupt gasification of volatile impurities adsorbed in the pores of the carbon particles. This phenomenon is observed invariably in a fluidized bed type regenerator which has separate drying and reactivating sections or chambers. The powdering also occurs if insufficiently devolatilized carbon particles are allowed to enter the reactivating chamber or to shortcut to the reactivating chamber without remaining in the drying chamber for a sufficient time period to be devotilized.
The drying chamber usually receives the spent activated carbon particles in a wet state continuously, dropped from a hopper through a feed pipe which opens into the drying chamber. Therefore, a considerable temperature drop occurs in the drying chamber in the areas where the wet carbon particles are dropped, and the wet carbon particles which have heavily adsorbed contaminant substances tend to pile up in lumps in the low temperature areas, preventing smooth fluidization of the carbon particles and clogging perforations in the gas dispersing plate at the bottom of the drying chamber. Such stagnation of the wet carbon particles can jeopardize the safe operation of the regenerator and often necessitates frequent interruptions in operation.
It is an object of the present invention to provide a fluidized bed type spent activated carbon regenerator which will overcome the afore-mentioned difficulties and problems and which will permit efficient continuous operation over a long period of time.